The invention relates to an X-ray detector which includes a sensor matrix and a scintillator arrangement.
X-ray detectors are employed for converting X-rays into light and/or into detectable charge carriers.
The X-rays emitted by an X-ray source in computed tomography (CT) systems and other imaging X-ray systems penetrate a patient to be examined and is attenuated in conformity with the varying density and chemical composition of the tissue or bone to be examined. The X-rays are converted into light in a scintillating material in the X-ray detector. The X-ray detector is typically composed of an anti-scatter grid, a scintillator arrangement which is situated therebelow and a sensor matrix which is situated therebelow again. The sensor matrix consists of a multitude of light-sensitive sensors, a single sensor also being referred to as a detector element or a channel. Exposure of the X-ray detector to X-rays, causes scattered X-ray photons and scattered radiation in the visible wavelength range, thus giving rise to crosstalk between neighboring detector elements or channels. In order to reduce such crosstalk that falsifies the primary X-ray image to be formed, the X-rays are made to pass through an anti-scatter grid which is focused onto the focus of the X-ray source.
It is thus achieved that the detection of the X-ray photons always concerns only the X-ray photons that are characteristic of the attenuation of the irradiated object.
In order to reduce the crosstalk component of the overall signal even further, the scintillator arrangement, being enclosed by light-reflecting external layers, is additionally segmented by X-ray absorbing shutters which are known as separators, so that scattered radiation (visible radiation or X-rays) that is incident at an angle is absorbed and does not reach the neighboring detector element. For conventional single-line detectors such separators are usually constructed as white epoxy resin layers or shutters of a black metal (Pb, WO, MO) that are coated so as to be reflective (TiO2). For X-ray detectors with integrated electronic circuitry the metal inserts offer a further advantage, that is, protection of the underlying semiconductor structures against X-rays.
WO 98/58389 describes an arrangement for the manufacture of large-area two-dimensional grids. A grid is then formed by the stacking of metal layers in which holes are etched. The openings of the resultant grid can be filled with phosphor or another scintillator material. The large-area grids are realized by way of puzzle-like connection of a plurality of sub-grids. The lithographic manufacture of the metal layer enables a high precision to be achieved. The superposed metal layers are fixed by means of pins that are to be inserted into appropriate holes.
Such a grid is manufactured with a great expenditure. The manufacture of each metal layer requires its own mask. Moreover, the insertion of scintillator blocks into grids thus constructed is intricate, because on the one hand the dimensions of the described grid are aimed at achieving a very high resolution while on the other hand the metal layers have steep edges impeding the insertion of metal blocks.
It is an object of the invention to provide an X-ray detector in which the crosstalk of the scattered radiation is reduced in an X-ray detector which is segmented in two directions in space, and can be manufactured with a high precision and in large numbers at an acceptable cost. It is also an object of the invention to provide a scintillator in which the underlying semiconductor structures are protected against X-rays.
The object is achieved in that there are provided a plurality of layers of wire elements that are spaced apart, are arranged so as to receive scintillators at least partly between the wire elements.
To this end, first a grid is formed from layers of wire, while using a kind of weaving technique. The wire is to be selected from metals having a high X-ray absorption, for example molybdenum or tungsten. A plurality of wire elements of corresponding length are arranged in a layer. A layer to be arranged on top thereof is preferably oriented at an angle of 90° relative to the wire elements of the underlying layer. Repeated superposition of such wire element layers yields a grid which is segmented in two directions in space.
In a preferred embodiment of the invention a respective scintillator element is inserted in each of the resultant holes.
Because all holes have the same dimensions, scintillator elements that can be pressed, for example, into these holes can be manufactured in a simple and inexpensive manner. This results in a scintillator arrangement in which the individual detector elements of the underlying sensor matrix are separated from one another by the wire elements.
This type of scintillator arrangement can be realized in many sizes, the scintillator arrangement nevertheless maintaining a high stability. The scintillator arrangement can be adapted to curvatures as they occur, for example in the case of CT arms.
Because of the small height of the scintillator arrangement it is not absolutely necessary to focus the grid thus formed onto the focus of a radiation source.
A combined anti-scatter grid and scintillator arrangement is obtained by manufacturing a grid of correspondingly larger dimensions from wire elements. It is then useful to focus the grid onto the focus of the radiation source. To this end, the distance between the wire elements in the successive layers is varied in conformity with the radiation angle. Thus, the spacing of the wire elements in the upper layers of the grid must be smaller than the spacing of the wire elements in the lower layers of the grid. In this context the terms upper and lower relate to the direction of incidence of the X-rays. This means that the layer which is arranged nearest to the radiation source has the smallest spacing of the wire elements and that the layer situated furthest from the radiation source, or nearest to the sensor matrix, has the largest spacing of the wire elements. The geometrical shape of such a grid opening is that of a cone having a square base.
As described above, scintillator blocks or scintillator elements are inserted into the lower part of the grid thus formed. This results in a combined grid which, in conjunction with the underlying sensor matrix, forms an X-ray detector of flexible dimensions that can be manufactured at low costs. The wire elements can be arranged so as to form a grid according to the invention with great precision.
The wire elements can also advantageously be made of a synthetic material containing substances absorbing X-rays. This woven grid, or the layers with wire elements, can also be manufactured by means of an injection molding process. The wire preferably has a round cross-section of, for example approximately 100 μm, but any other available cross-section can also be used.
The grid structure dimension of the present embodiment is assumed to be approximately 1.5 mm×1.5 mm. Because of the small height of such a scintillator arrangement focusing is not necessary for the time being, but can also be realized. The grid for the scintillator arrangement, or also a grid combined with an anti-scatter grid, may comprise plane or slightly curved opening sides and hence can be used for or adapted to all known detector techniques. A typical height of such a scintillator arrangement is approximately 4 mm, so that a significantly lower building height is required in comparison with an anti-scatter grid. Scintillator elements cut or pressed so as to be shaped as small cubes are inserted into the grid openings. The dimensions are, for example 1.4 mm×1.4 mm× 4 mm. Using these dimensions the precision of the structure is determined by the grid. After the insertion of the scintillator elements, the grid is provided with a white light-reflecting adhesive or lacquer, for example epoxy resin. The majority of the surfaces of the wire elements will thus reflect; the reflection could be less only at the areas of direct abutment of the wires against the scintillator cubes or elements. When the grid structure itself is coated so as to be reflective or when it is made of a white material, this drawback could be compensated.
A scintillator structure which is segmented in two directions in space can thus be efficiently manufactured. The requirements imposed as regards precision of the regularity of the elements could thus far not be satisfied by a cutting process performed for the scintillator segmentation. Even in the case of less severe requirements imposed as regards the individual dimensions of the scintillator elements, embedding in a grid forces the structure into a regular raster whose precision can be readily checked.
It is also possible to absorb radiation incident perpendicularly in the separating layers or separators formed by the wire elements, so that any semiconductor structure present at that area is protected.
It has been found in a further embodiment that the spacing of individual wire elements in a layer can be advantageously varied. X-ray detectors may have a different resolution so that, for example, a coarse resolution may occur in the edge zone of the X-ray detector and hence also in the edge zone of the anti-scatter grid. To this end, the wire elements in the edge zones should be situated at a distance from one another which is larger than the distance in the central zone in which the resolution of the X-ray detector is highest.
The arrangement of a plurality of successive layers with the same orientation of the wire elements offers the advantage that scattered radiation is absorbed for all angles of incidence. In the case of a regularly constructed grid, in which the orientation of the wire elements changes regularly, scattered radiation of a given angle of incidence could traverse the grid through the gaps present between the wire elements of the individual layers. When a given regularity in the orientation of the grid is avoided, accidental passage of scattered radiation of a given angle of incidence is precluded.
In order to ensure that the individual layers can be suitably stacked, the wire elements can be embedded in a synthetic material which is transparent to X-rays in such manner that each layer has plane surfaces. The thickness of the layers, however, should not exceed the diameter or the cross-sectional dimensions of the wire elements. The layers of wire elements can also be immersed in a liquid X-ray transparent auxiliary material without filled layers being formed. The grid is removed from the auxiliary material before this material hardens. The wire elements are thus bonded together. It has been found that a round cross-section of the wire elements is particularly attractive for such bonding of the wire elements, because the area of contact between the wire elements is particularly small so that a suitable bond can be formed. When the wire elements are welded or soldered together, a square or polygonal cross-section may be advantageous, because more material and hence more surface area is then available for bonding.
The high stability and the low tendency to oscillation of the resultant anti-scatter grid are particularly advantageous in an embodiment of the anti-scatter grid according to the invention. The flexibility in adapting the anti-scatter grid to the resolution of the X-ray detector also appears to be a major advantage over other scattered radiation absorbers.
The object is also achieved by means of an X-ray examination apparatus which includes an X-ray detector in which the anti-scatter grid and/or the scintillator arrangement includes layers of wire elements that are spaced apart.